The invention relates to a method of manufacturing an anisotropic oxidic permanent magnet having a hexagonal crystal lattice and uniaxial anisotropy. The invention also relates to an anisotropic oxidic permanent magnet manufactured by such a method.
Netherlands Patent Application No. 7,807,537 describes such a method which includes the steps of (a) providing a starting mixture of (i) iron oxide and/or iron compounds which form iron oxide upon heating with (ii) one or more oxides and/or compounds which form the corresponding oxides upon heating of the metals barium, strontium, and lead in quantities which form a magnet having a desired ferrite phase; (b) preferring the mixture in an oxygen-containing atmosphere having an oxygen a partial pressure related to the prefiring temperature chosen to form a stoichiometric product, (c) grinding the prefired products, (d) shaping the prefired product in the presence of a magnetic field, and (e) sintering the prefired product.
The sintered magnets manufactured by means of this known method comprise several phases in the range from the M to the W ferrite phase. To produce the desired product, a very carefully determined quantity of oxygen, dependent on the firing temperature, is present in the oven atmosphere during prefiring and final sintering. M-ferrite is to be understood to mean herein a ferrite phase having a stoichiometric composition defined by the formula EQU MeFe.sub.12.sup.3+ O.sub.19,
where Me is Ba, Sr, or Pb, and in which the atomic ratio of Me to iron is equal to 1:12. The easy axis of magnetization of M-ferrite is parallel to the crystallographic c-axis. Permanent magnets comprising this ferrite phase are produced on a large scale, for example, under the trade name "Ferroxdure".
W-ferrite is to be understood to mean a ferrite phase having a stoichiometric composition defined by the formula EQU MeA.sub.2.sup.2+ Fe.sub.16.sup.3+ O.sub.27,
where Me is Ba, Sr, or Pb and A.sup.2+ is Fe, Zn, Cu, Ni, Mn, Mg or mixtures hereof. This W-ferrite, in which the atomic ratio of Me to (A.sup.2+ +Fe.sup.3+) is 1:18, has a crystal structure which is related to that of M-ferrite. Except for Co.sup.2+ -W-ferrite, the easy axis of magnetization is parallel to the c-axis.
The object of the method described in Netherlands Patent Application No. 7,807,537 is to provide an anisotropic oxidic permanent magnet which, as compared with the known oxidic permanent magnets, has an increased saturation polarization J.sub.s in combination with an acceptable value of the intrinsic coercivity H.sub.cJ. Although this object appears to be achieved to a certain extent, the totality of the magnetic properties of the resulting permanent magnets may be improved. As moreover the manufacture is based on the idea that during the whole heating and cooling process a chemical equilibrium should be maintained between the gaseous phase and the solid phase by oxygen exchange, a rather complicated method is required which involves heating up, prefiring, final sintering, and cooling in the presence of an oxygen-containing atmosphere having a variable oxygen partial pressure which is continuously adjusted to the firing temperature according to a given logarithmic relationship.